Mobile machines, for example on-highway vehicles or off-highway construction or mining equipment can be steered by their operators to follow a desired path. Conventional steering systems utilize wheels that swivel around pivot points that are close to ends of associated axles or, in many cases notional axles, where a conventional axle does not exist. These conventional steering systems allow changes to made to the wheel orientation with only limited torque input required from a corresponding steering actuator. Even when the machines are stationary, it is still usually possible for the operator to turn a steering wheel and affect orientation of the wheels.
Other steering arrangements are possible. For example, with off-highway articulated machines (ATs), a front portion of the AT swivels or articulates in relation to a rear portion of the AT. With this configuration, the machine's axles can be rigid and a pivot arrangement is not required for individual wheels. The wheels on the AT's front axle simply take on a different orientation to the wheels on the rear portion of the machine, because the entire solid front axle is pivoted along with the entire front portion of the machine. As another alternative arrangement, tracked machines typically steer by commanding different track speeds on left and right sides of the machine.
In some of these latter applications (e.g., in AT applications), rotational effort to articulate the machine can be a significant issue. It is clear that, for example with the AT being stationary, as the forward portion articulates to the left (rotates counter clockwise when viewed from above) the left front wheel will be rotating backwards and the right front wheel will be rotating forwards. Even through the front axle is typically a driven axle on these ATs, this wheel speed difference is accommodated for by the differential, which, for the most part, operates as a conventional open differential.
In any of the situations discussed above, speed differences arise due to the forward (or reverse) curvilinear motion of the machine, and in some cases, like in the AT application, wheel speed differences can also arise due to the articulation action. Conventionally steered vehicles or machines may not have this second component to a significant degree. Wheels of a mobile machine may also experience speed differences due to a loss of traction.
A machine's differential may be provided with one or more clutches that alter the normally open behavior of the differential to selectively control the speeds of the wheels. That is, the differential clutch may be used to limit or override the normal behavior of the differential to reduce wheel spin (i.e., to reduce a disproportionate speed of one wheel on an axle during acceleration) or wheel slip (i.e., to reduce skidding of one wheel during deceleration). During acceleration, for example, if one of the wheels of a wheel set loses traction and starts to spin, an open differential will normally transfer torque from the non-spinning wheel to the spinning wheel (during acceleration to equalize torque between the wheels), thereby limiting the overall driving torque delivered to the wheels and actually increasing spinning. Similarly, during deceleration, if one of the wheels loses traction and starts to skid, the open differential will normally transfer torque from the non-skidding wheel to the skidding wheel, thereby limiting the overall braking torque delivered to the wheels and actually increasing skidding. The differential clutch, however, can override the differential to increase the amount of torque transmitted to the non-spinning or non-skidding wheel.
Some differential clutches permit partial clutch engagement to allow greater control over the amount of torque delivered to the wheels. However, in order for these clutches to be operated effectively without negatively impact steering, a knowledge of the true ground speed of each wheel may be important. Other machine control systems can also benefit from knowledge of the true ground speed of each machine wheel. Examples of such systems include Anti-lock Braking Systems (ABS), Dynamic Stability Control (DSC) systems, and various steering control systems.
Existing traction control systems generally compare estimated wheel speeds across an axle and activate the differential clutch if the wheel speed difference is beyond a predetermined threshold. However, such control systems do not fully account for natural wheel speed differences that occur while steering or turning (i.e., conventional control systems do not account for articulation-induced wheel speed differences). In addition, such estimates are generally needed during worst-case condition, where wheel speeds are expected to deviate from ground speed.
The disclosed machine display system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.